[Qemu-devel] [Bug 1211910] [NEW] Logical to linear address translation is wrong for 32-bit guests on a 64-bit hypervisor

[Nils Asmussen]

Public bug reported:

I run a 64-bit hypervisor in qemu-system-x86_64 (without KVM) and on top of 
that I have a 32-bit guest. The guest configures the code-segment to have a 
base of 0x4000_0000 and a limit of 0xFFFF_FFFF with paging disabled. Thus, if a 
logical address of e.g. 0xC000_0000 is used, it should be translated to 
0x0000_0000 (linear and physical), because of the overflow that happens.
But this does not happen with the described setup. Instead, qemu seems to 
calculate the logical to linear translation with 64-bit addresses so that no 
overflow happens. Consequently, the resulting address is 0x1_0000_0000 and this 
gets written to exitinfo2 in the VMCB structure. This causes trouble for 
hypervisors that expect the upper 32 bits of exitinfo2 to be 0 for 32-bit 
guests.

Note also that the exact same setup runs fine on real AMD machines with
SVM. That is, the upper 32 bits in exitinfo2 are always 0 because of the
overflow.

I've tested that with the latest development version of QEMU (commit
328465fd9f3a628ab320b5959d68d3d49df58fa6).

** Affects: qemu
     Importance: Undecided
         Status: New

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https://bugs.launchpad.net/bugs/1211910

Title:
  Logical to linear address translation is wrong for 32-bit guests on a
  64-bit hypervisor

Status in QEMU:
  New

Bug description:
  I run a 64-bit hypervisor in qemu-system-x86_64 (without KVM) and on top of 
that I have a 32-bit guest. The guest configures the code-segment to have a 
base of 0x4000_0000 and a limit of 0xFFFF_FFFF with paging disabled. Thus, if a 
logical address of e.g. 0xC000_0000 is used, it should be translated to 
0x0000_0000 (linear and physical), because of the overflow that happens.
  But this does not happen with the described setup. Instead, qemu seems to 
calculate the logical to linear translation with 64-bit addresses so that no 
overflow happens. Consequently, the resulting address is 0x1_0000_0000 and this 
gets written to exitinfo2 in the VMCB structure. This causes trouble for 
hypervisors that expect the upper 32 bits of exitinfo2 to be 0 for 32-bit 
guests.

  Note also that the exact same setup runs fine on real AMD machines
  with SVM. That is, the upper 32 bits in exitinfo2 are always 0 because
  of the overflow.

  I've tested that with the latest development version of QEMU (commit
  328465fd9f3a628ab320b5959d68d3d49df58fa6).

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